This invention is directed to electromagnetic rotating machines, and is more particularly directed to an assembly for holding brushes in position in an electrodynamic motor. The invention is more particularly concerned with a brush board and brush holder assembly that may be molded of a high performance plastic resin, with the brush board frame and brush holders unitarily formed for increased strength and durability under heavy duty operating conditions.
In DC motors of the type in which a rotary armature is supplied current through a rotary commutator, it is typical to mount the brushes in brush holders or brush boxes that are attached to a frame or support plate, i.e., a brush board, which is mounted to one end of the motor housing. This will typically have a center aperture to accommodate the commutator structure. The brush boxes or brush holders are separate items, usually made of steel or brass, and are mounted to the brush board by means of mechanical fasteners such as bolts. The carbon brushes are positioned in rectangular profile cavities or chambers, where brush springs urge the brushes to move inward (radially) to contact the contact bars of the commutator (or slip rings in the case of an AC motor).
A number of brush holder arrangements have been presented in the literature, and some of these are illustrated in connection with DC motors as shown in U.S. Pat. No. 5,208,499 to Barber et al., U.S. Pat. No. 5,004,943 to Gagneux, U.S. Pat. No. 4,694,214 to Stewart, Sr., and U.S. Pat. No. 4,293,789 to King. These all employ a single support plate with a number of brush holders mounted or formed on it, and each has a limited ability to withstand temperature loads. This construction can limit the motor to lighter duty operation. Also, in the motors of the prior art it has been difficult to replace worn brushes, and it is usually necessary to disassemble the brush board to achieve that, if it is even possible to do so.
Motors of this type are frequently used in electric vehicles, such as fork lifts and golf carts, where the motor is supplied with low voltage DC, typically 12 to 120 volts, and has a relatively high current draw, e.g., from several amperes to several hundred amperes or more. There can be a substantial voltage drop through the brushes and commutator, producing significant heating of the brush holder arrangement during normal operation.